1. Field of the Invention
The present invention relates generally to an automatic pulse plasma welding method and apparatus for forming a lap joint between membrane sheets, and more particularly to an automatic pulse plasma welding method and apparatus for forming a lap joint between membrane sheets, which can promptly form a lap joint between stainless steel membrane sheets that are arranged on the inner surface of a membrane-type liquefied natural gas storage tank and provided with horizontal and corrugated parts, using automatic plasma welding under optimal welding conditions.
2. Description of the Prior Art
Generally, welding is a method of joining two metal materials, that is, base metals, by heating the portions of the base metals to be melted or half-melted, or by fusing base metals together with a filler metal. Welding methods are classified into arc welding, gas welding, thermit welding, electroslag welding, electrobeam welding, etc. Arc welding is classified into a consumable arc welding method in which a working electrode is consumed and a non-consumable arc welding method in which a working electrode is not consumed, according to whether or not a working electrode is consumed. There are gas tungsten arc welding (GTAW) and plasma arc welding as representative non-consumable arc welding methods. Further, the non-consumable arc welding method may be classified into manual welding and automatic welding according to its unique properties and its purposes.
The gas tungsten arc welding is a method of joining base metals by using an inert gas, such as Ar, He, etc., as a shield gas and melting the base metals using arc heat created between a tungsten electrode, which is a non-consumable electrode, and the base metals. In this case, an additional filler metal can be provided and melted together with the base metals. Argon Ar or helium He which is an inert gas is used as the shield gas so as to prevent the base metals and the tungsten electrode from oxidizing, so the gas tungsten arc welding is also called tungsten inert gas (TIG) welding. This welding method can be applied to all welding positions, its arc is highly stable, and the quality of a weldment is excellent, so the welding method is used in welding of materials sensitive to oxidization and nitrification or welding requiring low heat input and high quality construction.
The plasma arc welding is a method using a powerful plasma jet, which is a special form of the TIG welding. In plasma arc welding, a plasma gas is additionally provided besides a shield gas, and a tungsten electrode is located within a water cooled contraction nozzle. Plasma arc welding is characterized in that the arc is contracted by the contraction nozzle to form a cylindrical shape, so the area of a part of the base metal to which arc heat is applied is constant even though a distance between the nozzle and the base metal changes. Consequently, plasma arc welding is almost the same as TIG welding except that concentration of arc is improved by the contraction nozzle.
Further, since a true constant current power source always supplies a constant current during a welding operation, the value of the current does not change according to the length of the arc. Therefore, the true constant current power source can be profitably applied to the GTAW and the plasma arc welding in which the length of arc is restricted.
However, current variation is required so as to control the size of molten puddle and penetration, such that a mechanical device for supplying 2-stage current while continuously changing it is developed. In this case, since an outputted current has a pulsating wave, the current is called a pulse current. Such a pulse welding method is frequently used for out-of position welding or first layer welding of a single-sided weldment, because the pulse welding prevents a molten metal from flowing and consequently, it is highly advantageous to the progress of proper welding.
In the prior art, of the above welding methods, the TIG welding method is used for a lap joint between membrane sheets due to difficulties such as the construction of equipment, control, signal transmission, etc. However, the TIG welding method is problematic in that it is difficult to obtain satisfactory welding results because the lap joint between membrane sheets has straight parts and curved parts, an aiming direction of a welding torch continuously changes, in particular, at the curved parts, and there is a processing error or an assembly error generated during a fit-up operation.
Additionally, the TIG welding method is problematic in that a welding speed is lower than that of the plasma welding method, working efficiency is decreased due to the low welding speed, and welding defects are frequently generated at intersecting parts between flat parts and corrugated parts of membrane sheets, or inflection regions of the corrugated parts.